x86: fix compile error with corruption checking disabled

Fix compile error:

 arch/x86/mm/init_32.c: In function 'mem_init':
 arch/x86/mm/init_32.c:908: error: implicit declaration of function 'start_periodic_check_for_corruption'

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>